The present invention relates to composite sheet materials for magnetic and electronic shielding, and products obtained therefrom such as envelopes and boxes used for transporting magnetic recording medium (media) and/or electronic parts or component(s), protection cases for magnetic cards, storage files for magnetic recording medium (media), covers for electric wires, wallpaper or wall-decorative materials, adhesive tapes, etc.
Following the commercial development of audio-articles, information recording media for music, images or voices have been recently developed as software for the aforementioned articles. It has brought about, in addition to the traditional use of records and audio-tapes, a wide use of video-tapes, video-discs and laser discs.
Triggered by the practical use of VLSI (Very Large Scale Integration), miniaturization of computers, such as microcomputers, personal computers, etc., has greatly progressed, and propagation of computers, both office use and household use, has been also promoted. This situation is gradually causing replacement of floppy discs for the traditional information recording media, for example, magnetic drums, magnetic discs, magnetic tapes for recording characters, letters, numerals, signs, symbols, drawings and so on.
Such rapid progress of the electronics industry is bringing about a new problem which is related to various disturbances caused by electromagnetic waves or magnetism.
For example when electromagnetic waves or magnetism emitted from a computer or its related instruments directly hit the essential part of other computers, operation of the hit computer is hindered and when a floppy disc as a memory of the magnetic disc is affected by magnetism, the stored information is disturbed and even rendered usefuless. Against such a background, the demand for materials for shielding aganst the effects of electromagnetic waves and magnetism is rapidly increasing.
Now, floppy discs, laser discs, audio-tapes, video-tapes, etc., are becoming popular not only in offices but also in households, and various problems have arisen as to their preservation and transporation, envelopes in which such articles are conventionally placed subsequent to manufacture are unsuitable for packaging and transporation with envelopes of this kind, it has been found that articles so packaged are broken during transporatation by inappropriate handling. In addition, a new problem observed at present is that recorded information is erased or disappears when it is subjected to magnetic fields in the course of transportation.
Electronic parts or components used in computers or in, electronic instruments such as measures, are more and more miniaturized and lightened, on one hand, and more readily incorporated in the ever increasing electronic instruments on the other hand. Speedy transportation of those electronic parts, as essential parts for the electronic instruments, to their desired location is very desirable. Those electronic parts are susceptible to damage or destruction due to static electricity encountered during the transportation thereof.
Whether such electronic instruments, or electronic parts used therein are boxed or installed in a room, there arises a need for shielding electronic noise emitted from those electronic instruments or electronic parts, as well as for shielding against the ingress of electronic noise, is arising at into the box or the room. A similar need exists for shielding electronic noises emitted from wires of power sources for those electronic instruments and from wires connecting those electronic instruments with related devices. The influence of electronic noise in those wires must be also avoided.
Magnetic cards are at the moment widely utilized as cash cards and also as credit cards, and are usually carried by the owner in a holder. When an owner turns on a television set or stereo after having inserted the holder magnetic lines emitted from the television set or the stereo may seriously affect the memory stored in those cards kept in the holder. Sometimes those cards becomes unusable.
Studies and experiments conducted in the past suggest various suitable materials for excluding or shielding electronic noise such as electromagnetic interference, static electricity interference, and radio frequency interference, for example, (1) a plastic housing thermally sprayed with metallic zinc; (2) a plastic housing coated with conductive paint; (3) a plastic housing to which is applied wet metallizing such as via chemical (electroless) plating or electro-plating; (4) a plastic housing to which is applied dry plating such as via vacuum evaporation, ion plating and spattering; and (5) plastics formed by mixing with electroconductive fiber such as fibrous metal and carbon fibers, or metal flakes such as aluminum flakes and nickel flakes. All of those materials have, however, proven unsatisfactory for a variety of reasons, as for example some were limited in their use, some were difficult to be manufactured, or required oversized and expensive machinery in manufacturing, and some others were not satisfactory because of their insufficient shielding effect.
There have been to find suitable materials other attempts which can be seen in this field. Most of the materials found or tried are made of plastics laminated with metal foil(s) or metal sheet(s), for example, aluminum foil(s). Sufficient shielding effects cannot, however, be expected from those laminated articles, particularly the magnetic shielding effect.